Method and apparatus for producing an improved anhydrous caustic product

ABSTRACT

An improved dehydration method and apparatus for producing an anhydrous caustic product having a water content of 0.5 percent by weight or less. Caustic feed is heated to a temperature from about 600* to 700* F. and the water content removed therefrom in a two-stage vaporization process wherein the first stage is conducted in an evaporating chamber maintained at an absolute pressure of from 2 to 20 inches of mercury and the second stage is conducted in a flash tank maintained at an absolute pressure of below 5 inches of mercury, and preferably below 2 inches of mercury.

United States Patent 1,883,211 10/1932 Wilson Inventor Stanley J. Macek South Holland, Ill.

App]. No. 843,676

Filed July 22, 1969 Patented Dec. 28, 197] Assignee Whiting Corporation METHOD AND APPARATUS FOR PRODUCING AN IMPROVED ANHYDROUS CAUSTIC PRODUCT 7 Claims, 3 Drawing Figs.

24 I, 2,17 VS, 48,4 MS, 31, 48 L; 23/185, 274, 276, 184, 306, 307

References Cited UNITED STATES PATENTS CENTRlFUGAL I SEPARATOR HTG. MED-22 Primary Examiner- Norman Yudkoff Assistant Examiner-J. Sofer Attorney-Guest, Lockwood, Greenwalt & Dewey ABSTRACT: An improved dehydration method and apparatus for producing an anhydrous caustic product having a water content of 0.5 percent by weight or less. Caustic feed is heated to a temperature from about 600 to 700 F. and the water content removed therefrom in a two-stage vaporization process wherein the first stage is conducted in an evaporating chamber maintained at an absolute pressure of from 2 to 20 inches of mercury and the second stage is conducted in a flash tank maintained at an absolute pressure of below 5 inches of mercury, and preferably below 2 inches of mercury.

PATENIEU uscza an SHEET 1 0F 2 {[-TO EJECTOR STEAM L} FLASHTANK 4| 5 PRODUCT R E m w w a w L 6 L w #1 E N 5 W T 4 O 4 4 8 5 1 T 6 2 7 WW 3 3 5 T I, 4/ 4 w I. v E 4 2 w F w Ill \CUJJHWAII G H F H M 2 G 3 6 :w 7 2 Um I. 2 2 2 5 F 2 D I E A NDI EE T CS H 23; CONDENSATE FEED INVENTOR STANLEY J. MACEK BY dWW ATTORNEYS.

METHOD AND APPARATUS FOR PRODUCING AN IMPROVED ANIIYDROUS CAUSTIC PRODUCT BACKGROUND AND DESCRIPTION OF THE INVENTION The present invention generally relates to an improved dehydration method and apparatus. More particularly, this invention is concerned with an improved method and apparatus for producing an anhydrous'caustic soda product having a moisture content of less than 0.5 percent by weight. As such, the present invention also finds advantageous utility in the recovery of anhydrous caustic potash.

Conventionally, anhydrous caustic soda having a moisture content of approximately 2 percent to 0.5 percent by weight is produced in evaporators having heat exchangers using heat transfer mediums which operate at temperatures of from approximately 600 F. to 700 F. Typically, diphenyl, diphenyl oxide, or a eutectic mixture of the two, are suitable: these heat transfer mediums are commonly sold under the trademark Dowtherm.

To obtain concentrated anhydrous caustic products having a moisture content of less than 0.5 percent by weight, the evaporator must beoperated at a temperature over 700 F. Dowtherm, however, cannot be economically used at the higher temperatures required to produce these low moisture content caustic products in conventional dehydration systems and accordingly, it has heretofore been necessary to use molten salt as the heat transfer medium in dehydration systems for producing an anhydrous caustic soda product having a moisture concentration of less than 0.5 percent by weight.

Typically, the molten salt used in such caustic dehydration systems is a mixture of potassium nitrate, sodium nitrite and sodium nitrate. At the temperatures employed, these salts are extremely corrosive, require more costly equipment and are significantly more hazardous to work with than systems using the Dowtherm heat transfer mediums.

The present invention provides a method and apparatus wherein heat transfer mediums which operate in the 600 F. to 700 F. temperature range can be used to produce an anhydrous caustic soda or potash product of a purity equal to that of the conventional molten salt systems. As such, the present invention enables the economic use of natural circulation evaporators and less costly materials handling equipment,

ln accordance with a specific embodiment of the present invention, a caustic liquor fed containing water and from about 40 percent to 80 percent sodium hydroxide by weight is heated to a temperature of between 600 F. and 700 F. and then directly discharged into an evaporating chamber maintained under a pressure of from 2 inches to 20 inches of mercury absolute. A liquid caustic soda discharge having a moisture content of from 0.5 percent to 2 percent by weight is recovered from the evaporating chamber and then fed into a flash tank which is maintained under a pressure below 5 inches of mercury absolute and preferably below 2 inches of mercury absolute. The anhydrous caustic soda having a moisture content of less than 0.5 percent is recovered from the flash tank.

It is, therefore, an important object of the present invention to provide an improved method and apparatus for obtaining an anhydrous caustic product (e.g., caustic soda or caustic potash).

Another object of the present invention is to provide an improved method and apparatus for producing an anhydrous caustic soda product having a moisture content of less than 0.5 percent by weight in a dehydration system which employs a heat transfer medium operating at a maximum temperature which is below 750 F.

Another object of the present invention is to provide an improved method and apparatus for recovering an anhydrous caustic soda having a moisture content of less than 0.5 percent by weight with a conventional natural circulation long-tube vertical evaporator.

Other and further objects of the present invention will be apparent from the following detailed description thereof taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a flow diagram illustrating one embodiment of the present invention;

FIG. 2 is a enlarged view, partly in section, of another embodiment of the present invention; and

FlG. 3 is a horizontal sectional view taken along lines 3--3 of FIG. 2.

Referring to the drawings, and with particular reference to FIG. I, and the numeral 11 generally designates a caustic soda concentrator embodying principles of the present invention. In this regard, it will be appreciated that while this preferred embodiment of the present invention is specifically described with reference to a caustic soda concentrator that the principles of this invention also find advantageous utility in other concentrators such as, for example, concentrators used to produce anhydrous caustic potash. As is shown, concentrators 11 includes an evaporator 12, a centrifugal separator 13, a flash tank 14, a steam ejector or booster l5 and a condenser 16. In the illustrated embodiment, the evaporator 12 is a conventional long-tube vertical evaporator comprising a generally cylindrical evaporating chamber 17 and a vertical heat exchanger 18.

A caustic liquor feed, which typically can contain water and 40 percent to percent NaOH is fed into the heat exchanger 18 at tube inlet side 21. A heat transfer medium, preferably a eutectic mixture of diphenyl and diphenyl oxide, is fed into the heat exchanger 18 at a shell side inlet 22 to heat the caustic liquor feed to a temperature of between 600 F. and 700 F. The condensed heat transfer medium is withdrawn from the heat exchanger 18 at the condensate outlet 23. After being heated in the heat exchanger 18, the caustic liquor feed is discharged from the tube outlet end 24 into the evaporating chamber 17 as is generally indicated at 25.

In the illustrated embodiment, the stream of heated caustic soda entering the evaporating chamber 17 is discharged directly into a deflector baffle 26 to facilitate separation of liquid from the vapors. Concentrated liquid caustic collects at the dished bottom 27 of the evaporating chamber 17.

In order to vaporize most of the water from the heated liquor feed in the chamber 17, evaporating chamber 17 is maintained under vacuum of between 2 and 20 inches of mercury absolute. In the illustrated embodiment, this vacuum is maintained by condenser 16.

With the caustic liquor feed heated to approximately 700 F., and the chamber 17 maintained under a vacuum of approximately 5 inches of mercury absolute, a concentrated liquid caustic discharge at a temperature of from 620 F. to 680 F. and having a moisture content of less than 2.0 percent, and as low as 0.5 percent, by weight, is recovered at the discharge outlet 28 of the evaporation chamber 17. This liquid caustic discharge is then carried by pipe 31 to the inlet 32 of flash tank 14.

As is shown in the embodiment illustrated in FIGS. 1 and 2, flash tank' 14 includes a dished top 33, a cylindrical body portion 34 and a conical bottom 35. Vapors are transmitted from thank 14 through a vapor outlet 36, provided in the upper end of body portion 34, and caustic product is discharged through product outlet 41 located in the conical bottom 35 as shown.

in accordance with an important aspect of the present invention, essentially of the residual moisture is vaporized from the liquid caustic discharge in the flash tank 14 by maintaining a pressure in flash tank 14 of less than 5 inches of mercury absolute. Preferably, the flash tank 14 is maintained under a pressure of less than 2 inches of mercury absolute. in the illustrated embodiment, the vacuum is drawn by the steam ejector or booster 15 which is connected by pipe section 37 to the vapor outlet 36. Steam having a pressure of p.s.i.g. is fed into the steam inlet 38 of the booster 15 at a suitable rate to achieve the desired operating conditions in flash tank 14. Typically, I00 p.s.i.g. steam could be supplied to booster 15 at a rate of between 33 and 1,700 lbs. per hour, depending on the amount of residual moisture in the recovered liquid caustic discharged fed into the flash tank 14, to maintain a pressure of approximately 2 inches of mercury absolute in the flash tank 14.

By maintaining the flash tank 14 under a high vacuum of less than 5 inches, and preferably less than 2 inches, of mercury absolute, most of the residual moisture is flashed out of the liquid caustic discharge fed into the tank 14 whereby the anhydrous caustic product recovered from the product outlet 41 of the flash tank 14 has a moisture content of less than 0.5 percent by weight.

The vapors flashed out of the heated caustic liquor feed in the evaporating chamber 17 are fed through the centrifugal separator 13 to remove or separate any entrained liquid caustic carried by such vapors. In the illustrated embodiment, this vapor-liquid separation is accomplished by directing the vapors in a whirling movement through the separator 13 (more fully described in connection with FIGS. 2 and 3). Vapors which exit centrifugal separator 13 are then fed through vapor line 42 to the condenser 16.

The steam ejector or booster is of a conventional design which includes a venturi 43 comprising a converging section 44 and a diverging section 45. As is shown, steam inlet 38 of the booster 15 is located adjacent the lower end of the converging section 44 and outlet 46 of the booster 15 is connected to the vapor pipe 42 as indicated at 47.

As shown in FIG. 1, end portion 48 of the vapor line 42 is connected to the vapor discharge outlet 49 of separator 13 and end portion 51 of vapor line 42 is connected to vapor inlet 52 of condenser 16. Condenser 16 can be constructed in a known manner and generally includes a cylindrical body portion 53 which is enclosed by a dished top 54 and a rounded bottom 55. A water inlet 56 is provided in the body portion 53 intermediate the top 54 and the bottom 55 and a condensate outlet 57 is located in the bottom 55. A vapor outlet 58 is located in the body portion 53 adjacent the top 54 for removing noncondensable gases from the condenser 16. The condensate is discharged through the condensate outlet 57 to a hotwell (not shown).

In the embodiment of the invention shown in FIG. 2, like reference numerals have been used to identify similar components. In this regard, it will be observed that the principal distinction between the FIG. 1 and FIG. 2 embodiments is that the FIG. 2 embodiment includes a separate condenser 16a in place of the booster 15 of the FIG. 1 embodiment. As is shown in FIG. 2, condenser 16a communicates with the vapor outlet 36 of flash tank 14 through a venturi 37a. Condenser 16a can be constructed in a known manner and generally includes a cylindrical body portion 53a which is enclosed by a dished top 54a and rounded bottom 55a. A water inlet 56 is provided in the body portion 53a intermediate top 54a and bottom 55a and a condensate outlet 57a is located in the bottom 55a. A vapor outlet 58a is provided in the body portion 53a adjacent the top 54a for removing noncondensable gasses from the condenser 16a. Condensate is discharged through the condensate outlet 57a to a hotwell (not shown).

As best shown in FIG. 2, the heat exchanger 18 includes a plurality of tubes 61 secured at the upper end 62 of the heat exchanger 18 to the shell side end wall 63. End wall 63 holds the upper ends of the tubes 61 in position to direct the liquor feed against deflector 26 and prevents the heat transfer medium from entering into the evaporating chamber 17 at the upper end 62 of the heat exchanger 18. A similar end wall (not shown) is located at the lower end 64 of the heat exchanger 18. If desired, a vent connection for the heat exchanger can be provided as indicated at 65.

The bottom 27 of the evaporating chamber 17 is provided with a sump 66 for collecting the concentrated liquid caustic and directing the liquid caustic to the discharge outlet 28.

Preferably, a thermometer or thermocouple socket 67 is provided on the outer wall 68 of sump 66 whereby the temperature of the liquid caustic discharge can be monitored. Normally, the level 71 of the liquid caustic in the evaporating chamber 17 is maintained well above the open top of the sump 66.

In the illustrated embodiment, the top 73 of the evaporating chamber 17 is slightly rounded and has a vapor outlet opening 74 communicating with the separator 13. If desired, opening 74 can be circular and generally concentric with the evaporating chamber 17. As shown, separator 13 is mounted to the top 73 of the chamber 17 above the opening 74 in a suitable manner.

Separator 13 includes a cylindrical wall portion 75, a dome portion 76 closing off the top of the separator 13 and a bottom wall portion 77 connected to the cylindrical wall portion 75 and secured to the top 73 of chamber 17. The bottom wall portion 77 is provided with a circular opening 78 which is eccentrically aligned with the opening 74 whereby vapors from the evaporating chamber 17 can easily flow into the separator 13.

As best shown in FIGS. 2 and 3, the centrifugal separator 13 includes a plurality of vanes 81 which extend upwardly from the bottom wall portion 77 and radially outwardly, in a generally flared pattern from the center of the evaporator 12 toward the cylindrical wall portion 75. If desired, a separator washout connection can be provided as indicated at 82.

In the present invention, the separator 13 is utilized for removing drops of liquid caustic product entrained with the vapors. This is accomplished by the whirling movement of the vapors through the separator 13 which is caused by the flared positioning of the vanes 81. The whirling movement results in the depositing of the entrained liquid caustic product on the vanes 81 or on the inner surface 82 of the cylindrical wall portion 75. Liquid caustic product expelled from the vapors is collected in a collecting trough 83 and removed by drain tubes 84:: and 84b which communicate with drain tube 84c and sump 66.

In the practice of the present invention as generally shown in FIGS. 1 and 2 caustic liquor feed containing water and from 40 percent to percent sodium hydroxide by weight is fed into the process inlet 21 of the heat exchanger 18 of the evaporator 12. Usually, the concentration of sodium hydroxide in the liquor feed is between 50 percent and 73 percent by weight. The heat transfer medium, preferably a eutectic mixture of diphenyl and diphenyl oxide sold under the trademark Dowtherm A, is introduced into the heat exchanger 18 through the process inlet 21 at a temperature between 600 F. and 700 F. THe heated caustic liquor feed is then directly discharged from the heat exchanger 18 into the evaporating chamber 17 where moisture is vaporized from the heated caustic liquor feed.

To enhance the vaporization of moisture from the heated feed liquor, evaporating chamber 17 is maintained at a pressure of from 2 inches to 20 inches of mercury absolute. In one preferred embodiment of the invention, the pressure in the evaporating chamber 17 is maintained at approximately 5 inches of mercury absolute. Typically, the vapors which are vaporized in the evaporating chamber 17 contain approximately 0.5 percent NaOH by weight. These vapors are passed through the separator 13 which removes from the vapors most of the entrained liquid NaOI-I. As shown in FIG. 2, the liquid NaOH extracted from the vapors is carried by drains 84a, 84b and 840 to the sump 66 at the bottom of evaporating chamber 17. The liquid caustic discharge recovered from the chamber 17 at the discharge outlet 28 has a moisture content of from approximately 0.5 percent to 0.2 percent moisture by weight. This liquid caustic discharge is then fed through pipe 31 to flash tank 14. Preferably, the temperature of the liquid caustic discharge is between 620 F. and 680 F. when it enters the flash tank 14.

Flash tank 14 is maintained at a pressure below 5 inches of mercury absolute and, preferably, at a pressure of approximately 1 inch of mercury absolute. At this reduced pressure, substantially all of the residual moisture in the liquid caustic discharge fed into flash tank 14 is flashed off the liquid caustic. As a result, the anhydrous caustic soda product recovered from flash tank 14 at the product outlet 41 contains less than 0.5 percent moisture by weight. Typically, moisture content of the anhydrous caustic product recovered from the flash tank 14 is from 0.1 percent to 0.2 percent by weight.

In the FIG. 1 embodiment, low pressure vapors from the flash tank 14 are fed into the booster 15, which is of conventional construction, and are discharged from the booster 15 into vapor line 42. Typically, steam having a pressure 100 p.s.i.g. is fed through the booster 15 at a rate sufficient to maintain the desired reduced pressure in flash tank 14.

Vapors from centrifugal separator 13 and from the booster 15 are then fed through vapor line 42 to the condenser 16 where they are condensed by the water entering the water inlet 56. Preferably, the water in the condenser 16 is between 75 and 90 F.

While in the foregoing specification, for purposes of illustrating the specific embodiments, many details have been set forth, it will be apparent to those skilled in the art that many of thesedetails can be varied without departing from the spirit of this invention. Accordingly, the present invention is to be limited only by the scope of the appended claims.

Iclaim:

1. An improved method of producing an anhydrous caustic product having a solids content of at least 99.5 percent by weight in a single-pass nonrecirculating evaporator system, said method comprising the steps of: heating a caustic feed liquor containing water and from 40 percent to 80 percent NaOH by weight in a heat exchanger to a temperature of from approximately 600 F. to 700 F; directly discharging the full flow of said heated caustic feed liquor into an evaporating chamber maintained under vacuum at a first pressure of from 2 inches to 20 inches of mercury absolute; recovering a liquid caustic discharge having a predetermined moisture content from said evaporating chamber; introducing the full flow of said recovered liquid caustic discharge from said evaporating chamber directly into a flash tank maintained at a second pressure blow said first pressure in said evaporating chamber, said second pressure also not being greater than 5 inches of mercury absolute and being sufficiently low to remove a sufficient amount of the residual moisture in said recovered liquid caustic discharge so as to provide an anhydrous caustic product having a solids content of at least 99.5 percent by weight; and, recovering the anhydrous caustic product from said flash tank.

2, The method as defined in claim 1 wherein said first pressure in said evaporating chamber is from 2 inches to 10 inches of mercury absolute.

3. The method as defined in claim 1 wherein said first pressure in said evaporating chamber is approximately 5 inches of mercury absolute. v

4. The method as defined in claim 1 wherein said second pressure maintained in said flash tank is less than 2 inches of mercury absolute.

5. The method as defined in claim 1 wherein said second pressure maintained in said flash tank is approximately 1 inch of mercury absolute.

6. The method as defined in claim 1 wherein said recovered liquid caustic discharge is introduced at a temperature of from approximately 620 F. to 700 F.

7. A nonrecirculating single-pass apparatus for producing an anhydrous caustic product having a solids content of at least 99.5 percent by weight, said apparatus comprising means adapted to heat a caustic liquor feed containing water and from 40 percent to percent NaOH by weight to a temperature of from approximately 600 F. to 700 F., an evaporating chamber, means for maintaining said evaporating chamber at a pressure of from 2 inches to 20 inches of mercury absolute, means for directly discharging the full flow of heated caustic liquor feed from said caustic liquor heater means into said evaporating chamber for vaporizing the water in said heated liquor feed to concentrate the NaOH in said caustic liquor feed and produce a concentrated liquid caustic discharge having a predetermined moisture content, means for recovering said concentrated liquid caustic discharge from said evaporating chamber, a flash tank, means communicating said flash tank with said evaporating chamber recovery means and adapted to directly transfer the full flow of said concentrated liquid caustic discharge form said evaporating chamber recovery means to said flash tank without further heating of said concentrated liquid caustic, and vacuum means operatively associated with said flash tank for maintaining said flash tank at a pressure lower than the pressure in said evaporating chamber and not greater than 5 inches of mercury absolute for vaporizing residual moisture in the liquid caustic discharge to produce said anhydrous caustic product having a solids content of at least 99.5 percent by weight.

Patent No.

CERTIFICATE OF CORRECTION Da d December 28, 1971 lnvent0r(s) Column Column Column Column Column Column Column Column Column Column Column Column (SEAL) Attest:

Stanley J. Macek line line line line line line line

line

line

line

line

line

EDWARD M.F LETC HER JR Attesting Officer It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

47, "fed" should read .-feed- 18, "concentrators" should read -concentrator- 60, "thank" should read tank 64, after "essentially" insert all 2, "discharged" should read discharge- 56, "gasses" should read -gases- 62, before "liquor" insert heated- 46, after "temperature" insert of 47, "'IHe" should read --'Ihe 2, after "Typically, insert the 38, "blow" should read --below 35, "form" should read from Signed and sealed this 9th day of January .1973

ROBERT GOTTSCHALK Commissioner of Patents F ORM PO-1OSO (10-69) USCOMM-DC 60376-P69 w 0.5 Govznnmsrn PRINTING OFFICE: I969 0-386-334, 

2. The method as defined in claim 1 wherein said first pressure in said evaporating chamber is from 2 inches to 10 inches of mercury absolute.
 3. The method as defined in claim 1 wherein said first pressure in said evaporating chamber is approximately 5 inches of mercury absolute.
 4. The method as defined in claim 1 wherein said second pressure maintained in said flash tank is less than 2 inches of mercury absolute.
 5. The method as defined in claim 1 wherein said second pressure maintained in said flash tank is approximately 1 inch of mercury absolute.
 6. The method as defined in claim 1 wherein said recovered liquid caustic discharge is introduced at a temperature of from approximately 620* F. to 700* F.
 7. A nonrecirculating single-pass apparatus for producing an anhydrous caustic product having a solids content of at least 99.5 percent by weight, said apparatus comprising means adapted to heat a caustic liquor feed containing water and from 40 percent to 80 percent NaOH by weight to a temperature of from approximately 600* F. to 700* F., an evaporating chamber, means for maintaining said evaporating chamber at a pressure of from 2 inches to 20 inches of mercury absolute, means for directly discharging the full flow of heated caustic liquor feed from said caustic liquor heater means into said evaporating chamber for vaporizing the water in said heated liquor feed to concentrate the NaOH in said caustic liquor feed and produce a concentrated liquid caustic discharge having a predetermined moisture content, means for recovering said concentrated liquid caustic discharge from said evaporating chamber, a flash tank, means communicating said flash tank with said evaporating chamber recovery means and adapted to directly transfer the full flow of said concentrated liquid caustic discharge form said evaporating chamber recovery means to said flash tank without further heating of said concentrated liquid caustic, and vacuum means operatively associated with said flash tank for maintaining said flash tank at a pressure lower than the pressure in said evaporating chamber and not greater than 5 inches of mercury absolute for vaporizing residual moisture in the liquid caustic discharge to produce said anhydrous caustic product having a solids content of at least 99.5 percent by weight. 